Ground electromagnetic instruments are a sort of instruments that calculate formation resistivity by measuring surface electrical field and magnetic field signals, and are widely applied in mineral exploration, oil and gas survey, subterranean water, and geothermal heat domains, etc. During field operation with a ground electromagnetic method, usually several measuring lines are arranged at an equal interval (the distance between two adjacent measuring lines is usually hundreds of meters) and several measuring points are arranged on each measuring line at an equal interval (the spacing between adjacent measuring points is usually hundreds of meters), and the instrument carries out measurement at the measuring points sequentially. The operation area of field operation with a ground electromagnetic instrument may be as large as several square kilometers or even tens of square kilometers.
In the field operation, the instrument carries out measurement for a time period at each measuring point, the time span of the measurement varies depending on the measurement method, and the volume of acquired data also varies. For example, with a magnetotelluric/audiomagnetotelluric (MT/AMT) method, data may be acquired at the measuring points for several or even tens of hours and the data volume may be hundreds of megabytes in one operation cycle; with a controlled source audiomagnetotelluric (CSAMT) method, data may be acquired at the measuring points for about 1 hour, and the data volume may be several megabytes in one operation cycle. In view that the instruments work for a long time, the operator usually wants to check the working state of the instruments in the working process of the instruments to ascertain whether the instruments operate normally. Therefore, the operator has to carry out patrol-check for the instruments and make corrections timely if any abnormality is found.
At present, the data storage media used by ground electromagnetic instruments are removable storage devices, such as CF cards and SD cards, etc. At each operation cycle is finished, the data is mainly read with either of the following methods: 1) removing the memory card and connecting it to a computer to read the data, and then reinstalling the memory card into the instrument after data reading; 2) connecting a computer through a data cable to the instrument and then reading the data, wherein, the data cable is usually a high-speed data transmission cable, such as USB cable or Ethernet cable, etc. The drawbacks of using a removable storage device include: the vibration resistance of such a detachable interface is usually inferior to the vibration resistance of fixed interfaces; since the storage device has to be removed and then reconnected, wearing damages to the connecting interface of the storage device may occur easily; in addition, since the terminals of the interfaces may be contaminated by clay, the electrical properties of the terminals of the interface may be compromised, i.e., the service life of the interface may be shortened. The same problem of removal/reconnection of the cable also exists when a transmission cable is used; consequently, loose contact may occur and the service life of the transmission cable may be shortened during long-time use in field operation.
Both methods require local operation, i.e., the operator has to go to the instrument and operate with a computer. Moreover, to check the working state of the instrument, the operator has to go to the instrument from time to time. Namely, the operator has to operate at the instrument in and after the working process of the instrument, resulting in manpower waste and degraded efficiency.